1. Field of the Invention.
The present invention relates to mass spectrometry and, particularly, to the confinement of ions within a cell during detection.
2. Description of the Prior Art.
Ion cyclotron resonance (ICR) is a known phenomena and has been employed in the context of mass spectroscopy. Essentially, this mass spectrometer technique has involved the formation of ions and their confinement within a cell for excitation. Ion signals may then be detected for spectral evaluation. An example of such a device is disclosed in U.S. Pat. No. 3,742,212, issued June 26, 1973 which is hereby incorporated by reference.
A later development to the above incorporated patent is Fourier Transform mass spectroscopy through which rapid and accurate mass spectroscopy became possible. This technique is disclosed in U.S. Pat. No. 3,937,955 issued Feb. 10, 1976 which is commonly owned with the present invention and which is also hereby incorporated by reference.
Both of the above-referenced patents employ ion trapping while the mass analysis is performed either by the measurement of the absorption of the applied excitation radio frequency of the ions at their resonance state or by direct detection of the cyclotron frequency of the excited ions. Both require trapping of ions, after formation, in an electrostatic DC trapping cell. In such conventional systems, the ions are formed using known techniques such as electron impact, Laser desorption, Cesium ion desorption, etc. The ions thus formed undergo a circular (orbital) motion known as cyclotron motion. This motion is due to the thermal energy of the ions and the applied magnetic fields of the known devices and is restricted in the directions orthogonal to the magnetic field of those devices as a result of that magnetic field. As is known in the art, these axes are referred to as the X and Y axes. The ion motion in the Z axis (that axis parallel to the magnetic flux lines) is restricted by electrostatic potentials applied to the trapping plates. The polarity of the ions that are trapped is determined by the polarity of the DC electric potential applied to the trapping plates.
It is well known to those skilled in the art that two significant problems that interfere with exact mass measurement in mass spectrometers arise from an electrostatic inhomogeneity due to the applied DC trapping potentials as well as the fluctuations of the magnetic field. In recent years, the use of a persistent type, high field, superconductive magnet with inherent field stability has ameliorated one of these factors of uncertainty from exact mass measurement. That is, magnetic field variation has been greatly reduced, if not eliminated. The other factor, the effect of the electrostatic trapping field, remains with known prior art devices.
As is known to those familiar with the art, the presence of the DC electrostatic trapping potential affects the frequency of the ion cyclotron motion. The major effect of this field is a shift of ion cyclotron frequency. This effect is nonlinear and mass dependent. This nonlinearity has made the task of ion cyclotron frequency to ion mass conversion very complex. To this is added the uncertainty about the value of the true trapping field and its stability within the cell and the nonlinear equipotentials in a conventional cell. With conventional instruments, the frequency to mass translation is done empirically by the use of an internal or external calibrant.